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Earthquakes • Shaking and trembling of the earth’s crust. • About 8000 occur every day or one every 11 seconds • Caused by plates sliding beside each other • Tsunami - earthquake on the ocean floor: causing waves to become greater than 20 meters high A strike-slip earthquake causes the plates to slide past one another. Normal plate movement is slipping in a downward motion. A thrust earthquake occurs when the plate moves in an upward motion. Seismic Waves • The three main types of seismic waves are: P waves, S waves, and L waves P Waves • Primary waves • Arrive first at the epicenter S Waves • Secondary waves • Move in up-down motion L Waves • Slowest moving seismic waves • Travel on top of Earth’s surface Measuring Earthquakes • Seismograph-measures and detects seismic waves • Richter Scale- a scale that allows scientists to determine earthquake strength based on many readings • 1-10 levels at which an earthquake is measured Above a 6 is very destructive • Epicenter - aboveground of origin; most violent shaking occurs at the epicenter WHAT INFLUENCES THE AMOUNT OF DAMAGE AN EARTHQUAKE CAUSES? - magnitude of the earthquake (how strong and fast the seismic waves are) - distance of location from epicentre of the earthquake - type of movement of the plates or along fault - local soil conditions (loose, solid, flexible, stiff) - construction type and quality of infrastructure